Electrically conducting cements containing epoxy resins and silver



Dec.- 18, 1956 H. WOLFSON ET AL ELECTRICALLY CONDUCTING CEMENTSCONTAINING EPOXY RESINS AND SILVER Filed March 25, 1952 MIX lNT/MA TEL YFLA AE S/L VEI? A/VD UL 7AM FINE PRzE'C/P/M TED \S/L VER M/X ARALD/TE MAIN COMPONENT AND CORRESPONU/NG cmmr AND ADD omega/v5 ALCOHOL.

A00 ARALD/ff" M/X Tc/AE a/eAoz/A LL y 70 (SM VER M/X 70/85 //v MORTARSTORE MIX rug? sroppmfo JAR H.WOLFSON G.E LLIO T ATTORNEY United StatesPatent Application March 25, 1952, Serial No. 27 8527 Claims priority,application Great Britain April 5, 1951 1 Claim. (Cl. 26032.8)

The present invention relates to improvements in electrically conductingcements, and to the application of the cement according to the inventionto the problem of fixing a semi-conducting crystal used for an electricrectifier or crystal triode, to a metal base or holder.

In the case of germanium rectifiers, it has been the usual practice tofix the germanium crystal to the base by plating and soldering. This,however, gives rise to difiiculties on account of the etchingtreatmentwhich must be applied to the germanium to produce a satisfactoryrectifier.

Best results are obtained if the crystal slice is etched beforeattachment to the base, as otherwise if the etching is applied aftersoldering, the presence of the brass and solder produces an undesirableeffect. It is also difiicult to solder the crystal slice on to the baseafter etching without contaminating the etched surface.

The principal object of the invention is to overcome these difficultiesby using an improved type of conducting cement for fixing the crystalslice to the base.

The invention accordingly provides an electrically conducting cementcomprising a thermosetting binding medium with which are incorporatedfine electrically conducting particles in such proportion that when thebinding medium sets, the said particles are brought into electricalcontact with one another throughout the whole mass, the binding mediumbeing of a type which adheres to the particles and occupies theinterstices between the particles, thereby holding the whole solidlytogether.

The invention also provides a method of making an electricallyconducting cement comprising intimately mixing together fine flakesilver and ultra-fine precipitated silver, and gradually adding to thesilver mixture a liquid thermosetting compound in sufficient quantity toform a very viscous paste in which all the silver particles are justcoated with the compound.

The invention further provides a method of making an electricsemi-conducting device which comprises fixing a semi-conducting crystalbody to a metal base by means of a thermosetting conducting cement ofthe kind specified above.

The invention will be described with reference to the accompanyingdrawing, in which Fig. 1 illustrates diagrammatically the preferredprocess of making the conducting cement according to the invention, andFig. 2 shows the stages in the process of fixing a crystal slice to ametal base using the cement.

The preferred formula for the conducting cement according to theinvention includes a thermosetting coating resin sold under theregistered trademark Araldite which is made up of a hardenable epoxyresin comprising the condensation product of 1chloro-2,3-epoxypropanewith 4,4 isopropylidene diphenol. The formula is 2,774,747 Patented Dec.18, 1956 The Araldite mixture should preferably be prepared by mixingthree parts of the main component with one part of the correspondingcatalyst to give a medium containing about 50% solids, and the diacetonealcohol should be added.

The flake silver and the ultra-fine precipitated silver should be firstintimately mixed. The silver mixture should then be placed in a mortar,and the Araldite mixture should be added gradually, working it into thesilver powder. The resulting mass should be a very viscous paste. Theprocedure is very similar to the determination of an oil absorption of apigment.

With the proportions given above, there should be just sufiicient of theAraldite medium to coat all the silver particles. 1

The conducting cement so prepared may be stored in a stoppered jar, andwill keep for three months at normal temperature. The process justdescribed is illustrated in Fig. 1.

The diacetone alcohol is used as a solvent for adjusting the viscosityof the mixture, and the proportion of this ingredient may be varied toproduce a suitable viscosity for the particular circumstances of the useof the cement. Some addition of this solvent to the mixture may benecessary from time to time to make up for evaporation.

It is very desirable that the ultra-fine precipitated silver used in theabove formula should not contain any appreciable proportion ofexcessively large particles. The preferred method of preparing it is asfollows:

680 grams of recrystallised silver nitrate are dissolved in 2 litres ofwarm distilled water in a 5-litre beaker. The temperature of thesolution should be about 50 C. 1.5 grams of sodium alginate aredissolved in 150 cc. of hot distilled water and the solution is mixedwith the first solution. Continuous stirring should be maintained duringthe mixing. 250 grams of sodium sulphite of laboratory reagent grade arethen dissolved in 1 litre of distilled water at a temperature of 50 to60 C., and the solution is added to the silver nitrate mixture. A verythick curdy precipitate of silver sulphite is formed. 200 cc. of 40%aqueous formaldehyde solution is then added, and the temperatureshouldbe adjusted to about C. Then 350 cc. of 0.880 ammonia solution isfinally added gradually. A considerable rise of temperature now occurs,and should be controlled so that it is kept just below C. After all theammonia solution has been added, the suspension should be kept between95 and C. for 30 minutes to ensure the completion of the reaction. Thesodium alginate reduces the rate of reduction of the silver sulphite,and excess of this reagent will prevent complete precipitation of thesilver. Owing to its fine state of subdivision, the silver is very dark,particularly when damp, and it does not settle out readily, and filtersslowly. The suspension should be filtered with a pump While hot andshould be washed with 2 litres of distilled water while still on thepump, followed by a litre of acetone. The product should be dried at 150C. overnight.

As shown in Fig. 2, in order to fix a germanium crystal to thecorresponding metal base or stub 1, a blob of the cement is placed onthe base, for example by dipping a wire 2 into the jar in which themixture is stored, and picking up a blob 3 on the end as shown in Figs.2 (a) and (b) and the crystal 4 is dropped on to the cement and pressedfirmly down with the tip of a glass rod 5 (Fig. 2 (0)), and then thewhole is baked in an oven 6 (Fig. 2 (a')) at about C. for 90 minutes.Shorter baking times can be used at higher temperatures. After baking,the crystal is found to be firmly fixed to the base, and cannot beremoved without splintering the germanium.

The cement appears to have almost the conductivity of block silver.

This conducting cement may evidently be used for joining any twoconductors, particularly when the use of plating, or solder, or the hightemperature associated with soldering, are precluded. Evidently,silicon, or other semi-conducting materials, could be mounted on metalbases with this cement.

The method of preparing the cement explained above gives what isbelieved to be the best combination of conductivity and adhesion. If thequantity of the Araldite medium were too small, the final cement wouldhave high conductivity, but would be friable or crumbly, with pooradhesion. If the quantity of the medium were increased, the mechanicalstrength and adhesion would be good, but the conductivity would be less.

In the cement, prepared as described, the silver particles are justcoated by the Araldite medium, which contains about 50% total solids.After polymerisation, 50% of the bulk of the medium is lost byevaporation of the solvent, and each silver particle is then no longercompletely coated, but the cement occupies the interstice between theparticles which are brought together in contact, yielding a conductingmaterial.

It should be pointed out that a rather unexpected result has beenobtained with this cement. Araldite is an extremely good electricalinsulator and the conductivity is not usually much aifected by theaddition of other matter. In the present case the electrical propertiesof the Araldite have been profoundly altered without destroying themechanical properties, by the introduction of the silver in suchquantity that the silver particles come into contact only after thebaking treatment.

It should be added that although Araldite is the preferred bindingcompound for the silver particles, other thermosetting compounds whichwill adhere satisfactorily to metals when set could be loaded up in likemanner with fine conducting particles in such proportion that when thebinding compound has set, the conducting particles are drawn together incontact throughout the whole mass.

While the principles of the invention have been described above inconnection with specific embodiments and particular modificationsthereof, it is to be clearly understood that this description is madeonly by way of example and not as a limitation on the scope of theinvention.

What we claim is:

An electrically conducting cement, comprising 27% fine flake silver, 46%ultra fine precipitated silver, 24.5% hardenable resinous reactionproduct of epichlorohydrin with a diphenol and 2.5% diacetone alcohol.

References Cited in the file of this patent UNITED STATES PATENTS1,901,391 Howard et al. May 23, 1933 2,137,428 Van Gcel et a1 Nov. 22,1938 2,173,249 Boer et al Sept. 19, 1939 2,280,135 Ward Apr. 21, 19422,324,961 StoflEel July 20, 1943 2,470,352 Holmes May 17, 1949 2,473,884Hein June 21, 1949 2,500,600 Bradley Mar. 14, 1950 2,506,130 Bain May 2,1950 2,570,856 Pratt et al. Oct. 9, 1951 FOREIGN PATENTS 554,972 GreatBritain a- July 28, 1943

